Voyager Saturn Science Summary

Index

Introduction
Saturn
The Rings
Titan
New Satellites
Other Satellites
The Magnetosphere

Introduction

The Voyager 1 and 2 Saturn encounters occurred nine months apart, in November
1980 and August 1981. Voyager 1 is leaving the solar system. Voyager 2
completed its encounter with Uranus in January 1986 and with Neptune in August
1989, and is now also en route out of the solar system.

The two Saturn encounters increased our knowledge and altered our
understanding of Saturn. The extended, close-range observations provided high-
resolution data far different from the picture assembled during centuries of
Earth-based studies.

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Saturn

Saturn’s atmosphere is almost entirely hydrogen and helium. Voyager 1 found
that about 7 percent of the volume of Saturn’s upper atmosphere is helium
(compared with 11 percent of Jupiter’s atmosphere), while almost all the rest
is hydrogen. Since Saturn’s internal helium abundance was expected to be the
same as Jupiter’s and the Sun’s, the lower abundance of helium in the upper
atmosphere may imply that the heavier helium may be slowly sinking through
Saturn’s hydrogen; that might explain the excess heat that Saturn radiates
over energy it receives from the Sun. (Saturn is the only planet less dense
than water. In the unlikely event that a lake could be found large enough,
Saturn would float in it.)

Subdued contrasts and color differences on Saturn could be a result of more
horizontal mixing or less production of localized colors than in Jupiter’s
atmosphere. While Voyager 1 saw few markings, Voyager 2’s more sensitive
cameras saw many: Long-lived ovals, tilted features in east-west shear zones,
and others similar to, but generally smaller than, on Jupiter.

Winds blow at high speeds in Saturn. Near the equator, the Voyagers measured
winds about 500 meters a second (1,100 miles an hour). The wind blows mostly
in an easterly direction. Strongest winds are found near the equator, and
velocity falls off uniformly at higher latitudes. At latitudes greater than 35
degrees, winds alternate east and west as latitude increases. Marked dominance
of eastward jet streams indicates that winds are not confined to the cloud
layer, but must extend inward at least 2,000 kilometers (1,200 miles).
Furthermore, measurements by Voyager 2 showing a striking north-south symmetry
that leads some scientists to suggest the winds may extend from north to south
through the interior of the planet.

While Voyager 2 was behind Saturn, its radio beam penetrated the upper
atmosphere, and measured temperature and density. Minimum temperatures of 82
Kelvins (-312 degrees Fahrenheit) were found at the 70-millibar level (surface
pressure on Earth is 1,000 millibars). The temperature increased to 143
Kelvins (-202 degrees Fahrenheit) at the deepest levels probed - - about
1,200 millibars. Near the north pole temperatures were about 10 degrees
Celsius (18 degrees Fahrenheit) colder at 100 millibars than at mid-latitudes.
The difference may be seasonal.

The Voyagers found aurora-like ultraviolet emissions of hydrogen at mid-
latitudes in the atmosphere, and auroras at polar latitudes (above 65
degrees). The high-level auroral activity may lead to formation of complex
hydrocarbon molecules that are carried toward the equator. The mid-latitude
auroras, which occur only in sunlit regions, remain a puzzle, since
bombardment by electrons and ions, known to cause auroras on Earth, occurs
primarily at high latitudes.

Both Voyagers measured the rotation of Saturn (the length of a day) at 10
hours, 39 minutes, 24 seconds.

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The Rings

Perhaps the greatest surprises and the most perplexing puzzles the two
Voyagers found are in the rings.

Voyager 1 found much structure in the classical A-, B- and C-rings. Some
scientists suggest that the structure might be unresolved ringlets and gaps.
Photos by Voyager 1 were of lower resolution than those of Voyager 2, and
scientists at first believed the gaps might be created by tiny satellites
orbiting within the rings and sweeping out bands of particles. One such gap
was detected at the inner edge of the Cassini Division.

Voyager 2 measurements provided the data scientists need to understand the
structure. High-resolution photos of the inner edge of the Cassini Division
showed no sign of satellites larger than five to nine kilometers (three to six
miles). No systematic searches were conducted in other ring gaps.

Voyager 2’s photopolarimeter provided more surprises. The instrument measured
changes in starlight from Delta Scorpii as Voyager 2 flew above the rings and
the light passed through them. The photopolarimeter could resolve structure
smaller than 300 meters (1,000 feet).

The star-occultation experiment showed that few clear gaps exist in the rings.
The structure in the B-ring, instead, appears to be variations in density
waves or other, stationary, forms of waves. Density waves are formed by the
gravitational effects of Saturn’s satellites. (The resonant points are places
where a particle would orbit Saturn in one-half or one-third the time needed
by a satellite, such as Mimas.) For example, at the 2:1 resonant point with
1980S1, a series of outward-propagating density waves has about 60 grams of
material per square centimeter of ring area, and the velocity of particles
relative to one another is about one millimeter per second. Small-scale
structure of the rings may therefore be transitory, although larger-scale
features, such as the Cassini and Encke Divisions, appear more permanent.

The edges of the rings where the few gaps exist are so sharp that the ring
must be less than about 200 meters (650 feet) thick there, and may be only 10
meters (33 feet) thick.

In almost every case where clear gaps do appear in the rings, eccentric
ringlets are found. All show variations in brightness. Some differences are
due to clumping or kinking, and others to nearly complete absence of material.
Some scientists believe the only plausible explanation for the clear regions
and kinky ringlets is the presence of nearby undetected satellites.

Two separate, discontinuous ringlets were found in the A-ring gap, known as
Encke’s Gap, about 73,000 kilometers (45,000 miles) from Saturn’s cloud tops.
At high resolution, at least one of the ringlets has multiple strands.

Saturn’s F-ring was discovered by Pioneer 11 in 1979. Photos of the F-ring
taken by Voyager 1 showed three separate strands that appear twisted or
braided. At higher resolution, Voyager 2 found five separate strands in a
region that had no apparent braiding, and surprisingly revealed only one small
region where the F-ring appeared twisted. The photopolarimeter found the
brightest of the F-ring strands was subdivided into at least 10 strands. The
twists are believed to originate in gravitational perturbations caused by one
of two shepherding satellites, 1980S27. Clumps in the F-ring appear uniformly
distributed around the ring every 9,000 kilometers (6,999 miles), a spacing
that very nearly coincides with the relative motion of F-ring particles and
the interior shepherding satellite in one orbital period. By analogy, similar
mechanisms might be operating for the kinky ringlets that exist in the Encke
Gap.

The spokes found in the B-ring appear only at radial distances between 43,000
kilometers (27,000 miles) and 57,000 kilometers (35,000 miles) above Saturn’s
clouds. Some spokes, those thought to be most recently formed, are narrow and
have a radial alignment, and appear to corotate with Saturn’s magnetic field
in 10 hours, 39.4 minutes. The broader, less radial spokes appear to have
formed earlier than the narrow examples and seem to follow Keplerian orbits:
Individual areas corotate at speeds governed by distances from the center of
the planet. In some cases, scientists believe they see evidence that new
spokes are reprinted over older ones. Their formation is not restricted to
regions near the planet’s shadow, but seems to favor a particular Saturnian
longitude. As both spacecraft approached Saturn, the spokes appeared dark
against a bright ring background. As the Voyagers departed, the spokes
appeared brighter than the surrounding ring areas, indicating that the
material scatters reflected sunlight more efficiently in a forward direction,
a quality that is characteristic of fine, dust-sized particles. Spokes are
also visible at high phase angles in light reflected from Saturn on the
unilluminated underside of the rings.

Another challenge scientists face in understanding the rings is that even
general dimensions do not seem to remain true at all positions around Saturn:
The distance of the B-ring;s outer edge, near a 2:1 resonance with Mimas,
varies by at least 140 kilometers (90 miles) and probably by as much as 200
kilometers (120 miles). Furthermore, the elliptical shape of the outer edge
does not follow a Keplerian orbit, since Saturn is at the center of the
ellipse, rather than at one focus. The gravitational effects of Mimas are most
likely responsible for the elliptical shape, as well as for the variable width
of the Huygens Gap between the B-ring and the Cassini Division.

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Titan

Titan is the largest of Saturn’s satellites. It is the second largest
satellite in the solar system, and the only one know to have a dense
atmosphere.

It may be the most interesting body, from a terrestrial perspective, in the
solar system. For almost two decades, space scientists have searched for clues
to the primeval Earth. The chemistry in Titan’s atmosphere may be similar to
what occurred in Earth’s atmosphere several billion years ago.

Because of its thick, opaque atmosphere, astronomers believed Titan was the
largest satellite in the solar system. Their measurements were necessarily
limited to the cloud tops. Voyager 1’s close approach and diametric radio
occultation show Titan’s surface diameter is only 5,150 kilometers (3,200
miles) - - slightly smaller than Ganymede, Jupiter’s largest satellite. Both
are larger than Mercury. Titan’s density appears to be about twice that of
water ice; it may be composed of nearly equal amounts of rock and ice.

Titan’s surface cannot be seen in any Voyager photos; it is hidden by a dense,
photochemical haze whose main layer is about 300 kilometers (200 miles) above
Titan’s surface. Several distinct, detached haze layers can be seen above the
opaque haze layer. The haze layers merge with the main layer over the north
pole of Titan, forming what scientists first thought was a dark hood. The hood
was found, under the better viewing conditions of Voyager 2, to be a dark ring
around the pole. The southern hemisphere is slightly brighter than the
northern, possibly the result of seasonal effects. When the Voyagers flew
past, the season on Titan was the equivalent of mid-April and early May on
Earth, or early spring in the northern hemisphere and early fall in the south.

Atmospheric pressure near Titan’s surface is about 1.6 bars, 60 percent
greater than Earth’s. The atmosphere is mostly nitrogen, also the major
constituent of Earth’s atmosphere.

The surface temperature appears to be about 95 Kelvins (-289 degrees
Fahrenheit), only 4 Kelvins above the triple-point temperature of methane.
Methane, however, appears to be below its saturation pressure near Titan’s
surface; rivers and lakes of methane probably don’t exist, in spite of the
tantalizing analogy to water on Earth. On the other hand, scientists believe
lakes of ethane exist, and methane is probably dissolved in the ethane.
Titan’s methane, through continuing photochemistry, is converted to ethane,
acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. The
last is an especially important molecule; it is a building block of amino
acids. Titan’s low temperature undoubtedly inhibits more complex organic
chemistry.

Titan has no intrinsic magnetic field; therefore it has no electrically
conducting and convecting liquid core. Its interaction with Saturn’s
magnetosphere creates a magnetic wake behind Titan. The big satellite also
serves as a source for both neutral and charged hydrogen atoms in Saturn’s
magnetosphere.

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New Satellites

Before the first Voyager encounter, astronomers believed Saturn had 11
satellites. Now they know it has at least 17 and possibly more. Three of the
17 were discovered by Voyager 1. Three additional possible satellites have
been identified in imaging data since the Voyager 2 encounter. (Three others
were discovered in ground-based observations.)

The innermost satellite, Atlas, orbits near the outer edge of the A-ring and
is about 40 by 20 kilometers (25 by 15 miles) in size. It was discovered in
Voyager 1 images.

The next satellite outward, Prometheus, shepherds the inner edge of the F-ring
and is about 140 by 100 by 80 kilometers (90 by 60 by 50 miles). Next is
Pandora, outer shepherd of the F-ring, 110 by 90 by 80 kilometers (70 by 55 by
50 miles). Both shepherds were found by Voyager 1.

Next are Epimetheus and Janus, which share about the same orbit – 91,000
kilometers (56,600 miles) above the clouds. As they near each other, the
satellites trade orbits (the outer is about 50 kilometers, or 30 miles,
farther from Saturn than the inner). Janus is 220 by 200 by 160 kilometers
(140 by 125 by 100 miles), and Epimetheus is 140 by 120 by 100 kilometers (90
by 70 by 50 miles). Both were discovered by ground-based observers.

One new satellite, Helene, shares the orbit of Dione, about 60 degrees ahead
of its larger companion, and is called the Dione Trojan. It is about 36 by 32
by 30 kilometers (22 by 20 by 19 miles). Helene was discovered in ground-based
photographs.

Two more satellites are called the Tethys Trojans because they circle Saturn
in the same orbit as Tethys, about 60 degrees ahead of and behind that body.
They are Telesto (the leading Trojan) and Calypso (the trailing Trojan). Both
were found in 1981 among ground-based observations made in 1980. Telesto is 34
by 28 by 26 kilometers (21 by 17 by 16 miles) and Calypso is 34 by 22 by 22
kilometers (21 by 14 by 14 miles).

There are three unconfirmed satellites. One circles Saturn in the orbit of
Dione, a second is located between the orbits of Tethys and Dione, and the
third, between Dione and Rhea. All three were found in Voyager photographs,
but were not confirmed by more than one sighting.

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Other Satellites

Mimas, Enceladus, Tethys, Dione, and Rhea are approximately spherical in shape
and appear to be composed mostly of water ice. Enceladus reflects almost 100
percent of the sunlight that strikes it. All five satellites represent a size
range that had not been explored before.

Mimas, Tethys, Dione, and Rhea are all cratered; Enceladus appears to have by
far the most active surface of any satellite in the system (with the possible
exception of Titan, whose surface was not photographed). At least five types
of terrain have been identified on Enceladus. Although craters can be seen
across portions of its surface, the lack of craters in other areas implies an
age less than a few hundred million years for the youngest regions. It seems
likely that parts of the surface are still undergoing change, since some areas
are covered by ridged plains with no evidence of cratering down to the limit
of resolution of Voyager 2’s cameras (2 kilometers or 1.2 miles). A pattern of
linear faults crisscrosses other areas. It is not likely that a satellite as
small as Enceladus could have enough radioactive material to produce the
modification. A more likely source of heating appears to be tidal interaction
with Saturn, caused by perturbations in Enceladus’ orbit by Dione (like
Jupiter’s satellite Io). Theories of tidal heating do not predict generation
of enough energy to explain all the heating that must have occurred. Because
it reflects so much sunlight, Enceladus’ current surface temperature is only
72 Kelvins (-330 degrees Fahrenheit).

Photos of Mimas show a huge impact crater. The crater, named Herschel, is 130
kilometers (80 miles) wide, one-third the diameter of Mimas. Herschel is 10
kilometers (6 miles) deep, with a central mountain almost as high as Mount
Everest on Earth.

Photos of Tethys taken by Voyager 2 show an even larger impact crater, named
Odysseus, nearly one-third the diameter of Tethys and larger than Mimas. In
contrast to Mimas’ Herschel, the floor of Odysseus returned to about the
original shape of the surface, most likely a result of Tethys’ larger gravity
and the relative fluidity of water ice. A gigantic fracture covers three-
fourths of Tethys’ circumference. The fissure is about the size scientists
would predict if Tethys were once fluid and its crust hardened before the
interior, although the expansion of the interior due to freezing would not be
expected to cause only one large crack. The canyon has been named Ithaca
Chasma. Tethys’ surface temperature is 86 Kelvins (-305 degrees Fahrenheit).

Hyperion shows no evidence of internal activity. Its irregular shape causes an
unusual phenomenon: Each time Hyperion passes Titan, the larger satellite’s
gravity gives Hyperion a tug and it tumbles erratically, changing orientation.
The irregular shape of Hyperion and evidence of bombardment by meteors make it
appear to be the oldest surface in the Saturn system.

Iapetus has long been known to have large differences in surface brightness.
Brightness of the surface material on the trailing side has been measured at
50 percent, while material on the leading side reflects only 5 percent of the
sunlight. Most dark material is distributed in a pattern directly centered on
the leading surface, causing conjecture that dark material in orbit around
Saturn was swept up by Iapetus. The trailing face of Iapetus, however, has
craters with dark floors. That implies that the dark material originated in
the satellite’s interior. It is possible that the dark material on the leading
hemisphere was exposed by ablation (erosion) of a thin, overlying, bright
surface covering.

Voyager 2 photographed Phoebe after passing Saturn. Phoebe orbits Saturn in a
retrograde direction (opposite to the direction of the other satellites’
orbits) in a plane much closer to the ecliptic than to Saturn’s equatorial
plane. Voyager 2 found that Phoebe has a roughly circular shape, and reflects
about 6 percent of the sunlight. It also is quite red. Phoebe rotates on its
axis about once in nine hours. Thus, unlike the other Saturnian satellites
(except Hyperion), it does not always show the same face to the planet. If, as
scientists believe, Phoebe is a captured asteroid with its composition
unmodified since its formation in the outer solar system, it is the first such
object that has been photographed at close enough range to show shape and
surface brightness.

Both Dione and Rhea have bright, wispy streaks that stand out against an
already-bright surface. The streaks are probably the results of ice that
evolved from the interior along fractures in the crust.

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The Magnetosphere

The size of Saturn’s magnetosphere is determined by external pressure of the
solar wind. When Voyager 2 entered the magnetosphere, the solar-wind pressure
was high and the magneto- sphere extended only 19 Saturn radii (1.1 million
kilometers or 712,000 miles) in the Sun’s direction. Several hours later,
however, the solar-wind pressure dropped and Saturn’s magneto- sphere
ballooned outward over a six-hour period. It apparently remained inflated for
at least three days, since it was 70 percent larger when Voyager 2 crossed the
magnetic boundary on the outbound leg.

Unlike all the other planets whose magnetic fields have been measured,
Saturn’s field is tipped less than one degree relative to the rotation poles.
That rare alignment was first measured by Pioneer 11 in 1979 and was later
confirmed by Voyagers 1 and 2.

Several distinct regions have been identified within Saturn’s magnetosphere.
Inside about 400,000 kilometers (250,000 miles) there is a torus of H+ and O+
ions, probably originating from water ice sputtered from the surfaces of Dione
and Tethys. (The ions are positively charged atoms of hydrogen and oxygen that
have lost one electron.) Strong plasma-wave emissions appear to be associated
with the inner torus.

At the outer regions of the inner torus some ions have been accelerated to
high velocities. In terms of temperatures, such velocities correspond to 400
million to 500 million Kelvins (700 to 900 million degrees Fahrenheit).

Outside the inner torus is a thick sheet of plasma that extends out to about 1
million kilometers (600,000 miles). The source for material in the outer
plasma sheet is probably Saturn’s ionosphere, Titan’s atmosphere, and the
neutral hydrogen torus that surrounds Titan between 500,000 kilometers
(300,000 miles) and 1.5 million kilometers (1 million miles).

Radio emissions from Saturn had changed between the encounters of Voyager 1
and 2. Voyager 2 detected Jupiter’s magnetotail as the spacecraft approached
Saturn in the winter and early spring of 1981. Son afterward, when Saturn was
believed to be bathed in the Jovian magnetotail, the ringed planet’s
kilometric radio emissions were undetectable.

During portions of Voyager 2’s Saturn encounter, kilometric radio emissions
again were not detected. The observations are consistent with Saturn’s being
immersed in Jupiter’s magnetotail, as was also the apparent reduction in
solar-wind pressure mentioned earlier, although Voyager scientists say they
have no direct evidence that those effects were caused by Jupiter’s
magnetotail.